Use of SWAT to model impact of climate change on sediment yield and agricultural productivity in Western Oregon, USA

Authors: Mueller Warrant, George; Phillips, Claire; Trippe, Kristin

Submitted to: Open Journal of Modern Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/2019
Publication Date: 4/28/2019
Citation: Mueller Warrant, G.W., Phillips, C.L., Trippe, K.M. 2019. Use of SWAT to model impact of climate change on sediment yield and agricultural productivity in Western Oregon, USA. Open Journal of Modern Hydrology. 9:54-88. https://doi.org/10.4236/ojmh.2019.92004.
DOI: https://doi.org/10.4236/ojmh.2019.92004

Interpretive Summary: This research evaluated the impact of climate change on soil erosion and crop yield in western Oregon using widely accepted crop production and climate models. Using ten different climate models that assume continued high emissions of greenhouse gases, western Oregon is forecast to have 4.7 to 10.3 °F of warming with average changes in annual precipitation ranging from decreases of 3.0 inches to increases of 8.6 inches. These changes are forecast to occur by the end of the 21st century. Many current crops will see yield losses as temperatures increase above optimum values, with moisture stress during warmer summers only adding to the problem. Current soil erosion rates of 2 to 7 tons per acre represent typical conditions seen in western Oregon. Soil erosion in some of the climate models consistently exceeded this range within the next 30 years, with nearly all of the models greatly exceeding it by 70 years in the future. Soil erosion became more severe as precipitation, temperature, or both increased over time. Much of the increases in soil erosion were due to increasing amounts of winter precipitation falling as rain, rather than snow, at elevations above 500 feet. Soil erosion rates in the worst case scenarios have the potential to strip topsoil from vulnerable landscapes (those possessing little to no ground cover during the winter) in less than a century. Vulnerable landscapes could result from: growing crops that are not adapted to higher temperatures and more extreme moisture stress, wildfires, foliar diseases, outbreaks of damaging insects, and deliberate harvest, tillage, or herbicide use by farm and forest managers.

Technical Abstract: Climate change predictions for the PNW include increasing temperatures, intensification of winter precipitation, and a shift from snow- to rain-dominant systems, all of which may increase the risk of soil erosion and threaten productivity. The agricultural/environmental model SWAT was run with climate predictions from the Coupled Model Intercomparison Project (CMIP5) “high CO2 emissions” scenario (RCP8.5) to study the impact of altered temperature and precipitation patterns on soil erosion and crop production in the Willamette River Basin of western Oregon. All 10 climate models produced substantial increases in sediment loading, with differences between averages for 2010-2019 and 2090-2099 ranging from 3.9 to 15.2 MT per ha among models. Sediment yield in the worst case model (CanESM2) corresponded to loss of 1.5-2.7 mm soil per year, or about one century to strip productive topsoil from the landscape. Most models predicted only small increases in precipitation (an average of 5.8% by the end of the 21st century) combined with large increases in temperature (an average of 0.05 °C per year). We found a strong correlation between predicted temperature increases and sediment loading, with a regression model combining both temperature and precipitation effects describing 79% of the total variation in annual sediment yield. A critical component of response to increased temperature was reduced snowfall during high precipitation events in the wintertime. SWAT characterized years with less than 20 mm of precipitation of snowfall as likely to experience severe sediment loss for multiple crops/landuses. Mid-elevation sub-basins that are projected to shift from rain-snow transition to rain-dominant appear particularly vulnerable to sediment loss. Analyses of predicted crop yields indicated declining productivity for many commonly grown grass seed and cereal crops, but increasing productivity for others. Adaptation by agriculture and forestry to more erosive conditions may thus need to include changes in management practices and species selection.